Flexography is one of widely used methods of printing onto foil, plastic film, corrugated board, paper, paperboard, cellophane, or even fabric. In fact, since the flexographic process can be used to print on such a wide variety of materials, it is often the best graphic arts reproduction process for package printing.
The anilox cylinder serves as the heart of the flexographic press. The use of an ink-metering anilox cylinder, which is engraved with a cell pattern, enables an even and fast ink transfer to the printing plate. The configuration of the cells in the anilox roller, the pressure between the rollers, and the use of a doctor blade mechanism control the amount of ink transferred to the substrate. The shape and volume of the cells are chosen to suit the anilox surface (chrome or ceramic), the doctoring system, the press capabilities, the printing substrate, and the image type (solid or halftone). Advances in anilox technology have resulted in laser-engraved ceramic anilox rollers offering tougher and longwearing rollers with greatly improved ink release characteristics compared to conventional mechanically engraved chrome roller technology.
Flexographic prints can be printed with a flexible printing plate that is wrapped around a rotating cylinder. The plate is usually made of natural or synthetic rubber or a photosensitive plastic material called photopolymer. It is usually attached to the plate cylinder with double-sided sticky tape. Flexography is a relief printing process, meaning that the image area on the printing plate is raised above the non-image area. The ink is transferred from the plate to the substrate in the printing nip. Flexography is a direct method of printing, i.e. the printing plate transfers the ink directly to the substrate. Due to improved registration, the most popular type of press is the central-impression (CI) press where printing units are arranged around a single central impression cylinder.
In general, the higher the speed of the press, the wider the press will be. When the press is wider and faster, the diameter of the anilox roller must be greater in order to prevent damage to the roller due to deflection and bending. A 50-inch (ca 127 cm) machine has a 6-inch (ca 15 cm) diameter anilox cylinder. For high speed presses the time necessary for the ink to travel from the anilox-plate nip to the printing nip is very short. Linear speeds in excess of 1800 ft/min (ca 0.549 km/min) are considered high speed for printing flexible substrates, and presses with the capability of printing at a linear speed of 3300 ft/min (ca 1 km/min) are now appearing on the market.
The linear speed of 3300 ft/min (ca 1 km/min) is equal to a linear velocity of 35 miles per hour (ca 56.3 km/hr), and conventional plates and the double-sided sticky tape will eject from the press at this speed. In place of plates and double-sided sticky tape, direct laser engraved elastomer sleeves are used for printing at these velocities. The usual chambered doctor blade has a two-inch gap between the blades, and the dwell time for this distance at 3300 ft/min (ca 1 km/min) is less than the time of a high speed shutter on a 35 mm camera. During that interval, the air present in the cells must be displaced with ink, and the air must be cycled out from the chamber. At linear speeds up to 2300 ft/min (ca 0.701 km/min), normal motors can be used; however, at linear speeds over 2300 ft/min water-cooled motors are preferred.
Many printers require inks and coatings to be printed at high speeds in order to improve the cost effectiveness of their operations. Flexographic printing linear speeds generally range up to 2000 ft/min (ca 0.609 km/min), and that speed can be expected to increase. At increasing linear speeds, for example greater than 1200 ft/min (ca 0.366 km/min), and especially 1800 ft/per minute (ca 0.549 km/min), the printability of the ink begins to deteriorate and print defects can be observed. This defect can be described as randomly distributed, irregularly shaped missed areas of printing. These defects are believed to result from the inability of the ink to wet out the surfaces of the printing plate and/or substrate, or from the distinct mechanistic demands associated with a high speed printing press configuration as discussed in the above paragraphs.
In opposite to flexography the gravure printing is an example of intaglio printing so that the image area is etched or engraved into a printing cylinder and is below the non-image area. The un-etched (not engraved) areas of the cylinder represent the non-image areas. The cylinder rotates in the ink fountain and the excess of ink is wiped off the cylinder by a flexible steel doctor blade. The ink remaining in the recessed cells forms the image by direct transfer to the substrate (paper or other material) as it passes between the plate cylinder and the impression cylinder nip.
Gravure inks are fluid inks with a very low viscosity that allows them to be drawn into the engraved cells in the cylinder and then transferred onto the substrate. Flexographic and gravure inks are very similar and the basic constituents are essentially the same.
The transfer of ink to the substrate may be one of the most important factors affecting the quality of the final printed product. Due to dynamics of linear high-speed presses, conventional inks used for slower speeds will breakdown at high speeds, creating print defects. Any print defect will negatively affect productivity and the inherent printing advantages of using linear high-speed presses.
Typical flexographic/gravure printing inks contain resins, solvents, colorants, and additives. The resins include rosin esters, polyamides, polyurethanes, nitrocellulose, and others. The solvents used in flexographic/gravure inks are for example: alcohols, esters, glycol ethers, hydrocarbons and other solvents.
It is known that one of the factors important for good printability is the surface tension of printing ink, and it is commonly accepted that low surface tension is necessary for good ink spreading and substrate wetting. For example, solvent-based flexographic inks have inherently low surface tension because of the solvents used (e.g., alcohols, esters, ethers, etc).
Good printability may be controlled by the appropriate balance of interfacial properties at all interfaces created on the press during printing process. For example, in flexographic printing the surface energy of printing plates cannot be too low. On low-energy surfaces the ink may not form a continuous ink film, or, even if formed, the ink film may break very easily. In both cases, the print obtained will be defective because some areas of the image will not be covered with ink, e.g., pinholing. Another cause of pinholing may be uneven ink lay on the printing plate in the printing nip—due to film splitting in the nips (ink filaments) and inadequate ink film leveling.
Ink film exiting the anilox roll-printing plate nip undergoes splitting via cavitation and filament formation. As the ink is carried further away from the nip, the surfaces of the anilox cylinder and plate cylinder continue to separate, the cavities expand vertically and ink filaments are formed between the cavities. The subsequent cavities and filaments are formed as long as ink continues to emerge from the nip during printing. The filaments elongate and become thinner as they continue their travel away from the nip. The rate at which filaments elongate, and then rupture and level, depends on the printing speed, viscoelastic properties of ink, anilox roll and plate characteristics, ink surface tension, ink film thickness, etc. The ink surface tension and gravitational forces as well as developed surface-tension-gradient effects (Marangoni flow, which arise during drying) will decide about leveling effects—ink film flatness.
As a result of filament formation and break-up the surface of wet ink film exiting the nip is irregular (wave and/or worm-like pattern) and will tend to smooth during the process of leveling. Leveling is a process of eliminating surface irregularities of a continuous ink film under the influence of the ink's surface tension. It is an important step in obtaining a smooth, flat and uniform ink film. The factors that resist leveling are viscosity, elasticity and surface tension gradient (responsible for upward flow). The process of leveling depends on many parameters such as: the ink film thickness, extent and frequency of surface irregularities, ink surface tension, viscosity etc. However, the surface tension and viscosity of the ink play a major role:
                              l          s                ~                              γ            l                    η                                    (        1        )            
where: ls is the leveling speed; η is the ink viscosity and γ1 is surface tension of ink.
The leveling speed increases with increasing surface tension of the ink and decreasing ink viscosity. During drying, ink properties will change due to loss of solvent by evaporation—increase of viscosity (increased resistance to leveling) and surface tension change. The details of changes during drying and impact on coating defects are occurring on very short timescales during printing and specific models have been not been successfully implemented for complex systems. Two particular fundamental studies of note are the work of Weidner et. al., Journal of Colloid and Interface Science, 179 pp. 66-75 (1996) and Yiantsios and Higgins, Physics of Fluids, 18 pp. 082103-1 to 082103-11 (2006). Both these studies attempt to model phenomena in films in order to describe the potential outcomes during the drying and relationship between compositional changes and surface tension within a simulated coating. There are, however, no descriptions describing how the knowledge of compositional impact on changes in surface tension can be used to create a preferred state of film performance.
Flexographic inks may contain ingredients of differing surface tension and volatility (solvent blends). In such systems a local surface tension gradient may be formed during drying (solvent evaporation—change in ink composition and evaporative cooling) of the ink film. Local ink flow connected to the surface tension gradient would counteract leveling forces due to surface tension and gravity.
The term leveling is often used by printers and ink makers to describe ink spreading on the substrate. It should be emphasized that spreading and leveling are two different processes. Though the surface tension of an ink or coating, γ1, plays an important role in both processes, the effect of surface tension on ink spreading is opposite to its effect on leveling. To achieve good spreading, the surface tension of the ink should be as low as possible; for good leveling, the surface tension should be as high as possible. In practical applications a compromise usually has to be reached regarding the 1 value to achieve optimum leveling and spreading.
The management of surface tension of inks has primarily focused on achieving proper surface tension for wetting within the print process during image transfer as well as for the substrate of which the final printed film resides. Wetting is usually focused at maintaining lower surface tension which can be at odds with desire to maintain high surface tension to insure print performance.
The inventors of the present invention discovered that formulation design which takes into account how the surface tension evolves during drying can lead to good print performance, particularly at high speeds. In high speeding printing, to have good print performance may be a challenging task as the time given to the ink film surface to level (traveling time from the anilox roll-plate cylinder nip to the printing nip) is very short, about 0.015 s (assuming printing speed of 2000 fpm and the distance between nips of 6 inches). The inventors of the present invention, however, found that good print performance can be achieved by the appropriate ink formulation. Ink that show good leveling properties should have low viscosity and maximal high surface tension, and should not develop (or to a minimal extent only) the surface tension gradient during ink drying (as long as the ink is in the liquid form). The values of all of the above parameters should be optimized and to be in the range acceptable by flexographic printing process.